Siloxane based materials are often prepared by copolymerization of a siloxanyl alkyl ester of methacrylic acid with other acrylate, methacrylate or itaconate monomers in many known formulations. Such compositions can exhibit excellent oxygen permeability, due in large measure to the use of the siloxane material. It is desirable to increase the siloxane content in order to increase oxygen permeability. However, such increase often decreases hardness values, sometimes giving difficulty in machineability, dimensional stability, and other values.
The art has recognized that introducing fluorine containing groups into contact lens polymers can also significantly increase oxygen permeability. These materials can range from soft to semi-rigid and often require the use of special wetting agents or surface treatments. Siloxanes, on the other hand, which increase oxygen permeability, sometimes have problems with biocompatibility and protein buildup.
Other contact lenses are known which are based on telechelic perfluorinated polyethers. Such lenses can have good oxygen permeability, but are often relatively soft; sometimes causing difficulty in using conventional machine techniques known to the art.
Still other contact lenses having good oxygen permeability have resulted from various mixtures and copolymerized materials of organosiloxanes and fluorinated components, as in U.S. Pat. No. 4,433,125. Here again, the materials sometimes exhibit low levels of hardness.
In many of the above contact lens materials, methacrylates, acrylates and itaconates are used to increase hardness, while still retaining optical properties which are necessary for contact lens use.
In general, oxygen permeability has often been increased in hard or rigid gas permeable contact lenses in the past, by the addition of silicone and/or fluorine-containing materials which often require the use of hardening agents in order to maintain hardness values desired.